Uninterruptible power supplies (UPSs) are commonly used to provide power to critical equipment that must not experience even short duration brownouts or blackouts. For example, computer servers computer networks, telecommunications electronics and medical devices are often powered by an uninterruptible power supply.
A UPS device typically has an AC-DC-AC converter and backup battery that is activated in case the alternating current (AC) line power is temporarily disconnected or falls below a voltage threshold. Some UPS devices have multiple UPS modules (connected in parallel), with each module having an AC-DC-AC converter. Multiple-module UPS devices are commonly used in the information technology (IT) industry.
In multiple-module UPS devices, it is necessary to distribute the load (i.e., output current) equally among the modules. Typically, equal load distribution requires measurement of the output current from each module, and appropriate feedback control of each module. For example, the DC bus voltage of each module can be adjusted so that each module has the same output current. While this method is effective, output current sensors are relatively expensive. Also, each module is subtly different, even when built with nominally identical components. Consequently, each module must be controlled with slightly different feedback gain in order to obtain accurately equal output current.
FIG. 1 shows a conventional multimodule UPS device. Although 3 modules are shown, the device can have 2, 4, or more modules. Modules 10a 10b 10c are identical, but typically have slightly different operating characteristics due to variation in component characteristics (e.g. on resistance or threshold voltage of MOSFET switches). Output current sensors 12a, 12b, and 12c measure the output current of each module. The output current measurements are used by a main controller 14 to adjust the operation of each module 10a, 10b, and 10c. Typically, the DC bus voltage in each module 10a, 10b, and 10c is adjusted in order to control the output current from each module 10a, 10b, and 10c. 
The output current sensors 12a, 12b, and 12c are typically Hall effect sensors. A disadvantage of the device of FIG. 1 is that a Hall effect sensor must be provided for every module. Hall effect sensors are relatively expensive, multiple Hall effect sensors significantly increase the cost of the UPS device.
Also, variations in the operating characteristics of each module can complicate efforts to accurately control the output current of each module, and can result in inaccurate output current control.
It would be an advance in the art of UPS devices to provide a multimodule UPS device with control of the output current of each module without requiring a current sensor for each module. Such a UPS device would have a significantly reduced cost and high performance.
Also, it would be an advance in the art to provide a UPS device that can compensate for variations in module characteristics. Compensation of UPS module variations can provide improved current balancing and more accurate output current control.